Sulfide minerals of the Hitachi deposits and their comparison with those of the Besshi deposit. Studies on sulfide minerals in metamorphosed ores of the Besshi and Hitachi copper deposits. 2.:Studies on Sulfide Minerals in Metamorphosed Ores of the Besshi
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Pyrite, chalcopyrite and sphalerite assemblage is the majority of regional metamorphosed ores of the Fudotaki deposit of the Hitachi mine. By contact metamorphism, pyrrhotite is formed from pyrite (desulfurization of pyrite) in the Fujimi and Irishiken deposits of the same mine.<BR>In contact metamorphosed ores, monoclinic pyrrhotite+hexagonal one having 47.3-47.5 atomic % Fe with or without pyrite assemblages are frequently observed. The hexagonal pyrrhotite is sometimes composed of two or three phases with different Fe contents. Polytypes and Fe contents of pyrrhotite may reflect the T-fs<SUB> 2</SUB> environments of the retrogressive metamorphism. Euhedral (sometimes porphyroblastic) pyrite coexists frequently with pyrrhotite. This association may indicate that these two minerals were once in equilibrium at the climax of the contact metamorphism. FeS contents of sphalerite are generally higher in the rims by 1-3 mole % than those in the central parts of grains. Exsolution dots of chalcopyrite and/or pyrrhotite are always observed in sphalerite. Adding the amounts of exsolved dots to measured FeS contents, the FeS contents of sphalerite associated with pyrite and pyrrhotite become to 18-20 mole % FeS in the rims of grains, which correspond to those on pyrite-pyrrhotite-sphalerite solvus occurring below 600°C. Thus the exsolution dots may have appeared with decreasing temperature along the monoclinic pyrrhotite-hexagonal pyrrhotite fs<SUB> 2</SUB> buffer.<BR>The desulfurization of pyrite by the contact metamorphism is not so strong in the Fujimi and Irishiken deposits of the Hitachi mine, showing the marked contrast to that in levels deeper than 26 L of the Besshi deposit, where the desulfurization is nearly complete. The marked difference of FeO/MgO ratios for silicates of rocks close to the massive ores between the Hitachi and Besshi deposits may explain this contrasted desulfurization. FeO contents for silicates become, in effect, poorer along the desulfurization of pyrite in the Besshi deposit. A role of Fe-Mg silicates for acceptor of sulfur was suggested.<BR>The appearance of exsolution dots in sphalerite in the Fujimi and Irishiken deposits, and the non-appearance of them in levels deeper than 26 L of the Besshi deposit may be due to the contrasted T-fs<SUB> 2</SUB>, environment in the retrogressive metamorphisms between these two deposits.
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